Splits the downloads area 60/40 with a new info pane that adapts to the
selected download's status: header (filename, user, size, status, save
path, bitrate, length) plus queued-specific fields (queue position, peer
slots, peer upload speed) or live progress/speed/ETA when transferring.
Plumbs queue position via a new ClientOperation::PlaceInQueueUpdate so
PlaceInQueueResponse messages from peers reach DownloadStore, and threads
search-time metadata (bitrate, length, peer slots/speed) onto Download
through a new download_with_metadata entry point.
Move the Vec<Download> and the download lifecycle methods (add, remove,
lookup, status update, pause, resume, remove-if-queued) into a new
download_store module. ClientContext keeps thin delegating shims so
peer/listen, peer/download_peer and the existing client tests carry on
unchanged. Client::pause_download / resume_download / remove_queued_download
now delegate one-liners. Client::process_failed_uploads keeps the
read-then-write lock dance but the inner filtering is the new
collect_failed_tokens helper.
Adds dedicated unit tests for the store's behavior (add/get/remove,
pause+resume, queued-only removal, failed-token collection) so the
state-machine logic is exercised independently of Client.
michel